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As private space travel is starting to become a reality a major question is where does space begin? This is an important question for companies like Virgin Galactic which promise to take customers who can afford the ticket to the outer boundary of space for a short amount of time before returning to Earth. The question is important because it demonstrates what will be one of the key bench marks for the success of private sector space travel.
The first thing to understand is that in reality there is no clear defining line where the Earth’s atmosphere ends and space begins. It is similar to the way that boundaries between countries or states aren’t “real” unless arbitrarily decided the atmosphere really just fades out becoming space.
As of right now there is an official boundary to space called the Karman line. This boundary is about 62 miles above sea level or roughly 100 km above sea level. This number is not the true boundary but was chosen as the official boundary by the International Aeronautic Federation. The IAF is the main body that governs all international laws on Aeronautics and Space exploration.
The Karman Line is named after Theodore von Karman. He posited the 100 km distance because it is the height at which aircraft can not be sustained by lift unless the travel at orbital velocity. When we consider that it makes sense as the human boundary for space and space exploration. However the real boundary of the atmosphere and space is less clear. If we were trying to be more accurate the Karman line is way off. It is only 20 km into the fourth closest layer of the atmosphere, the Thermosphere. The Space Station is also in this part of the atmosphere. The outermost layer is the exosphere which is as high as 10,000 km.
In the end the boundary of space is something that is not official. It is different for each country. In the United States we consider an astronaut anyone that has traveled above 80 km above sea. This shows that trying to set a boundary that is the exact beginning of space can seem to be a futile exercise. What we do know is that at some point the Earth’s atmosphere ends and space begins. The truly scientific answer would be to say that the atmosphere doesn’t end but mingles with the interstellar medium. Studying the interaction can lead to better and cheaper space travel.
We’ve recorded many episodes of Astronomy Cast, including one about Hubble. Check it out, Episode 88: The Hubble Space Telescope.